The purpose of the proposed research is to investigate the nature of virus induced cell fusion. This process will be studied with mutants of Herpes simlex virus (HSV) which causes cells to fuse during a productive viral infection. Wild type HSV does not produce cell fusion, but mutants can be isolated that do. Our approach in determining the molecular basis of ciral-induced cell fusion will be to genetically separate the cell fusion process from other events in the cell, and then to biochemically determine which membrane molecules are involved in the process. For the genetic separation, a large number of HSV mutants causing cell fusion will be isolated and characterized by complementation tests. Biochemical studies will be used to compare the synthesis of new membrane molecules in mutant and wild type viral infections, and to compare their incorporation into cell plasma membranes.